The invention relates to an electrophotographic method where electrostatic charge images of identical shape but opposite sign are generated on both sides of a transparent, highly insulating foil, pigment being deposited on both sides of the foil by means of oppositely charged developers.
Electrophotography utilizes the local variation of the conductivity of a flat photosemiconductor in reaction to light for generating images (Ullmans Encyklopadie der technischen Chemie, 3rd edition, volume 14 (Munich-Berlin 1963) page 678). Electroradiography is a special kind of electrophotography. While electrophotography utilizes light rays for the recording, electroradiography utlizes X-rays or other directly ionizing rays. (German Offenlegungsschrift No. 26 41 067.) Ionography is another special kind of electrophotography for recording X-ray images. In ionography, a latent image of the radiogram is formed as a distribution of the electric charge on an insulating surface rather than another selenium or photoconductor. The latent image is generated by collecting ions on the surface of an insulating foil which is suspended in front of an electrode of an ionization chamber. These ions are formed by radiation in a layer of a suitable gas which fills the space adjoining the foil. The latent image generated by the electric charge pattern can be made visible (developed) in various ways which are customarily used in electrophotography, (German Offenlegungsschrift No. 24 31 036 which corresponds to U.S. Pat. No. 3,963,924.)
The ionographic method described in U.S. Pat. No. 3,963,924 German Offenlegungsschrift No. 24 31 036 utilizes ionizing radiation which passes through an object to be imaged and which subsequently passes into an ionization chamber. The ionization chamber contains a layer of a gas, at least some atoms of which have a high absorption coefficient for X-rays. The gas layer is bounded by a pair of electrodes which sustain an electric field in the chamber. The ions produced in the gas layer are collected on the surface of a transparent insulating foil. In a modified version of this method, the foil is centrally arranged in the ionization chamber so that positive ions are collected on one side and an equal charge of negative ions is collected on the other side, the ions of opposite charge keeping each other in position as a result of their force of attraction, the net load on the foil being almost zero. It is important that the foil is held exactly in such a position that the opposite charges obtained on both sides of the foil are equal. The correct position is usually situated in the vicinity of the geometrical center of the gas layer. Both surfaces of a foil thus charged can be developed by means of some known method, for example, development by powder or liquid or by introduced or deposited substances with optically active properties.
Direct absorption of X-rays in a gas in the vicinity of the recording layer produces pairs of ions which are separated by an applied electric field, so that ions of the same charge polarity are collected on the recording layer. In the ionization chamber shown in FIG. 8 of the German Offenlegungsschrift 24 31 036, a number of charge pairs are formed by irradiation. After the irradiation is completed, negative charges are present on one side of the foil and positive charges are present on the other side of the foil. The number of such charge pairs amounts to half the number of charge pairs originally formed, because the positive partners of the charge pairs formed on one side of the foil proceed to the cathode, while the negative partners of the charge pairs formed on the other side of the foil proceed to the anode and are lost as far as the recording process is concerned. For the sake of comparison it is assumed that the method known from German Offenlegungsschrift No. 24 31 036 produces an optical density amounting to 1 on a single foil. This assumption wil be described further below.